The use of a multiply pulsed copper halide laser to significantly anneal out the damage to silicon solar cells caused by a simulated space radiation environment is investigated. Preliminary experiments demonstrate that the amount of damage can be decreased by 41% as measured by the maximum power generated

Pivirotto, T. J.

English

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ANNEALING RADIATION DAMAGED SILICON SOLAR CELLS 
WITH A COPPER HALIDE LASER 
Thomas J. Pivirotto 
Jet Propulsion Laboratory 
Pasaden a, Calif or ni a 
ABSTRACT 
A multiply pulsed copper halide laser  has been used t o  significantly 
anneal out the damage t o  s i l icon solar ce l l s  caused by a simulated space 
radiation environment. 
amount of damage can be decreased by 41% as measured by the maximum power 
generated. 
Preliminary experiments have demonstrated t h a t  the 
INTRODUCTION 
The performance of s i l icon solar ce l l s  is  degraded i n  the radiation 
environment of space by energetic particles such as electrons, protons, 
neutrons and ions. 
reduced power generation capability of the irradiated solar array. 
overcome t h i s  degradation effect  on mission performance the solar panels are 
generally b u i l t  large enough t o  supply the spacecraft power required a t  the 
end of the mission. 
panels a t  launch and an excess i n i t i a l  o u t p u t  power. 
This degradation increases w i t h  time and resul ts  i n  a 
To 
This sol-ution resul ts  in overly large and heavy solar  
An a l ternat ive approach t o  this problem would be a provision for  
periodically annealing the damaged ce l l s  i n  situ. 
accomplished w i t h  a h i g h  efficiency laser which has an output wave length t h a t  
i s  readily absorbed by silicon. 
weight,  simplified power conditioning, and extended mission l i f e  times. 
Some previous experimental studies of the ab i l i t y  of various lasers t o  
anneal radiation damaged s i l icon solar ce l l s  have been reported1.2. 
lasers used were: 
absorption length (a- l )  in si l icon of about lo4 pm; 2) YAG with a A o f  1.06 pm 
and a a-l of 80 pm; and 3) Ruby w i t h  a h of 0.694 pm and an a- l  of 10 pm. 
Performance recovery, as measured by the maximum generated power, of from 20 
t o  80% was observed. 
T h i s  could be conveniently 
T h i s  approach could resul t  in a lower launch 
The 
1) C02 w i t h  a wave length ( A )  of about 10 pm and an 
Because of the encouraging resul ts  of the previous work i t  was f e l t  that  
1) the small absorption length for  the CHL radiation i n  s i l icon;  
a copper halide laser  (CHL)  should be investigated fo r  t h i s  application 
because of: 
The research described i n  t h i s  paper was carried out a t  the Je t  
Propulsion Laboratory, California Ins t i tu te  of Technology, under NASA Contract 
NAS 7- 100 
277 
https://ntrs.nasa.gov/search.jsp?R=19810009043 2020-03-21T09:01:20+00:00Z
2) t h e  p o t e n t i a l  s c a l a b i l i t y  t o  very h igh  average powers; and 3) i t s  
demonstrated h igh  e f f i c i ency .  The absorpt ion length, i n  s i l i c o n ,  of t h e  two 
CHL output  wave lengths (511 and 578 nm) i s  about 1 1.1. 
l eng th  w i l l  become more important as t h e  th ickness o f  t h e  s i l i c o n  c e l l s  
decreases i n  t h e  fu ture.  
t ime requ i red  t o  f u l l y  anneal l a rge  s o l a r  panels i n  space and t h e  h igh  
e f f i c i e n c y  w i l l  reduce t h e  s i z e  and weight o f  t h e  l a s e r  power supply and heat 
r e j e c t  i on system. 
This  shor t  absorpt ion 
The p o t e n t i a l l y  h igh  average power w i l l  decrease t h e  
EXPERIMENTAL APPROACH 
A p re l im ina ry  experiment, u t i l i z i n g  e x i s t i n g  equipment, was developed t o  
t e s t  t h e  f e a s i b i l i t y  o f  t h e  CHL f o r  s o l a r  c e l l  annealing. 
these experiments had a maximum pulse energy o f  1.5 mj, a pu lse w id th  o f  about 
20 ns, peak pulse i n t e n s i t y  of 75 kW/cm2 ( w i t h  60% of t h i s  i n t e n s i t y  a t  511 nm 
and t h e  remainder a t  578 nm), a beam diameter o f  about 1 cm, a pu lse 
r e p e t i t i o n  r a t e  va r iab le  between about 10 and 30 kHz, an average power o f  15 
watts and an e f f i c i e n c y  o f  about 1%. 
As shown i n  F igure 1, t h e  l a s e r  beam was r e f l e c t e d  i n t o  t h e  v e r t i c a l  
plane, f o r  convenience, then focused w i t h  an achromatic lens t o  the  des i red  
spot diameter a t  t h e  s o l a r  c e l l  surface. The s o l a r ’ c e l l  was housed i n  a small 
vacuum chamber t o  prevent ox ida t i on  o f  t h e  ho t  s i l i c o n  and t o  p a r t i a l l y  
simulate t h e  heat t r a n s f e r  cond i t ions  i n  space. The vacuum chamber was 
mounted on a motor dr iven,  t r a v e r s i n g  platform. This  p l a t f o r m  could sweep t h e  
c e l l  through t h e  focused beam i n  t h e  X d i r e c t i o n  a t  a pre-determined speed and 
move i n  t h e  Y d i r e c t i o n  w i t h  pre-determined step sizes. 
e n t i r e  c e l l  surface could be t rea ted  w i t h  an approximately un i form energy 
density. 
The CHL used i n  
I n  t h i s  way t h e  
RESULTS 
Pre l im inary  t e s t s  o f  t h i s  system were performed on c e l l s  manufactured by 
Spectrolab Inc. (K6 3/4, BSF, BSR, MLAR). 
c e l l s  were measured th ree  times: f i r s t l y ,  as received from Spectrolab; 
secondly, a f t e r  i r r a d i a t i o n  w i t h  1 Mev e lec t rons  t o  a f luence o f  1015 e/cm2 
and a room temperature cure; and then a t h i r d  t ime a f t e r  annealing w i t h  t h e  
CHL. Typical r e s u l t s  a re  shown i n  Figs. 2 and 3. I n  bo th  o f  these 
experiments t h e  spot s i z e  was 0.17 cm w i t h  a spot overlap, i n  t h e  Y d i rec t i on ,  
o f  2/3. The average energy dens i ty  i n  t h e  spot was 0.013 j/cm2. The average 
power was 4.3 wat ts  and t h e  pulse r e p e t i t i o n  r a t e  was 14.3 kHz. For t h e  
r e s u l t s  o f  Fig. 2, t h e  t rave rs ing  speed was 0.16 cm/sec, r e s u l t i n g  i n  a t o t a l  
appl ied energy dens i ty  of about 500 j/cm2 on t h e  c e l l  surface. 
corresponding data f o r  t h e  r e s u l t s  o f  Fig. 3 are 0.13 cm/sec and 690 j/cm2. 
The I - V  c h a r a c t e r i s t i c s  o f  t h e  
The 
The r e s u l t s  shown i n  Fig. 1 and 2 c l e a r l y  i n d i c a t e  t h a t  t h e  damaged c e l l s  
were p a r t i a l l y  annealed i n  these p re l im ina ry  experiments. The shape o f  t h e  
c h a r a c t e r i s t i c  was a l t e r e d  by t h e  annealing process i n  t h e  experiment o f  Fig. 
2, bu t  t h e  performance recovery was subs tan t ia l .  I n  Fig. 3 it i s  seen tha t ,  
although t h e  performance recovery i s  not  as great  as t h a t  shown i n  Fig. 2, t h e  
shape o f  t h e  c h a r a c t e r i s t i c  was retained. These r e s u l t s  are i n  general 
agreement w i t h  those obtained w i t h  a YAG and Ruby l a s e r  and repor ted on i n  
Ref. 1 and 2. 
2 78 
DISCUSSION 
Once t h e  f e a s i b i f i t y  o f  us ing a CHL f o r  annealing r a d i a t i o n  damaged 
s i l i c o n  s o l a r  c e l l s  was establ ished by these p re l im ina ry  experiments, an 
improved experimental set-up was developed. This  new set-up includes: 
o s c i l l a t o r - a m p l i f i e r  system w i t h  a 25 wat t  average power and 2.5 mj/pulse 
capab i l i t y ;  2) near-optimum o p t i c a l  elements i n c l u d i n g  an unstable resonator 
which produces a near-diffraction-limited beam f o r  b e t t e r  focusing a t  t h e  
so la r  c e l l  surface; and 3)  an automatic t r a v e r s i n g  and indexing system w i t h  a 
much wider range o f  speed and step s i z e  and a l a r g e r  capacity. With t h e  new 
set-up t h e  de f i c ienc ies  o f  t h e  p re l im ina ry  set-up have been overcome and t h e  
parametric range, such as spot diameter, t rave rs ing  speed and amount o f  spot 
overlap, has been g r e a t l y  increased. 
Add i t iona l  experiments w i t h  t h i s  set-up should i n d i c a t e  t h e  amount o f  
so la r  c e l l  performance recovery which could be expected i n  an operat ional  
environment. With these improvements i n  experimental equipment and procedure, 
it i s  expected t h a t  t h e  CHL annealing process can be made more e f f i c i e n t  and 
t h a t  t h e  s o l a r  c e l l  performance can be increased t o  near i t s  p re- rad ia t ion  
l e v e l  e 
1) an 
CONCLUSIONS 
It has been dem 
r a d i a t i o n  damaged s i  
power, due t o  r a d i a t  
experiments w i t h  the  
p o t e n t i a l  o f  t h e  CHL 
ns t ra ted  t h a t  a Copper Hal ide Laser can be used t o  anneal 
i con  s o l a r  c e l l s .  To date 41% o f  t h e  loss o f  maximum 
on damage, was recovered by annesl ing w i t h  a CHL. Future 
improved experimental set-up should es tab l i sh  t h e  
f o r  t h i s  app l i ca t ion .  
REFERENCES 
1. K i r kpa t r i ck ,  A. R.; Neal, W. R.; Minnucci, J. A.; "Annealing o f  Radiat ion 
Damage i n  S i l i c o n  Solar  Cel ls" ,  Phase I Report, March 15, 1977, 
S imulat ion Physics, Inc., Bedford, Mass. 
2. Minnucci, J. A.; Matthei, K. W.; K i r kpa t r i ck ,  A. R.; " I n -S i tu  Annealing 
o f  Space Radiat ion Damage", Record o f  t he  13th Photovol ta ic  Spec ia l i s t s  
Conference, IEEE, Pg. 586 t o  589, (June 1978). 
279 
I i  I )  
8 cm FOCAL LENGTH ACHROMATIC LENS j 1 . 
@ D I AMRE R 
ADJUSTMENT 
I !  
\ i  
\ I  
1 1  
VACUUM CHAMBER 
\ I I  
-Y X 
Figure 1 .  Schematic of Solar Cell Annealing Experiment 
160 
140 
120 
E" 100 
r-' 
B 
PI 2" 
60 
40 
20 
0 
TEMPERATURE CURE 
0 1 0 0 2 0 0 3 0 0 4 a l 5 0 0 6 0 0  
VOLTAGE, mv 
Figure 2 .  Effect of Annealing 
Radiation Damaged Silicon Solar Cells 
with a Copper Halide Laser 
180 I I I I I I 
160 
140 
120 
E" 100 
z CL:
2 80 
60 
40 
20 
0 
TEMPERATURE CURE 
0 I 100 200 300 400 500 
600 
VOLTAGE, mv 
Figure 3.  Effect of Annealing 
Radiation Damaged Silicon Solar Cells 
with a Copper Halide Laser 
2 80 


Annealing radiation damaged silicon solar cells with a copper halide laser

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